With the advancement of miniaturization, high capability and functional improvement of electronic equipment, wiring boards are required to be suitable for high density installation. In order to satisfy such requirement, wiring boards are made multilayered, an insulating layer is thinned, a via diameter is made small, and a circuit is made to have narrow pitches.
As the technology for achieving such advancement, to make a metal layer thin has been known effective and there have been proposed methods for obtaining circuit substrates with a thin metal layer by forming a thin film of a metal on the synthetic resin film surface by vacuum evaporation or sputtering, and forming a metal layer with a prescribed thickness on the film by electroplating, and some of the methods are employed practically. However, the methods have a problem that the adhesion strength of the film surface and the metal thin film is low or that the heat resistance of the adhesion is low. Further, in order to avoid such a problem, there are methods comprising steps of forming, on the surface of a synthetic resin film, a thin film of a metal, which is different from a metal to form a circuit and has excellent adhesion property to a synthetic resin film, and then forming a metal with which a circuit is formed. Even by such methods, not only sufficient adhesion strength between the film surface and the metal layer cannot be obtained but also there occurs a problem in the electric resistance stability in circuits owing to the lamination of two or more kinds of metals. Further, there are other problems that the metal layers formed by a combination of thin film formation by vacuum evaporation, sputtering or the like with a plating method have pin holes and uneven layer thickness.
On the other hand, in order to obtain a laminate with a thin metal layer, it is supposed to be proper to use a thin metal foil. However, when the metal foil is thin, the metal foil ruptures owing to the insufficient strength and causes a problem of wrinkling at the time of lamination and therefore it is required for the metal foil to have a thickness of at least 10 μm in order to stably manufacture the laminate.
The present invention aims to solve such problems and provide a laminate for a printed wiring substrate and a multilayer printed wiring board which have high heat resistance, wiring patterns with narrow pitches, vias with a small diameter, insulating layer having uniform thickness and stable adhesion between the metal layer and the synthetic resin film, and which contribute to miniaturization, high capability and functional improvement of electronic equipment.